Embodiments of the invention relate to mud return systems used in the oil production industry. More particularly, embodiments of the invention relate to a novel system and method for lifting mud returns to the sea surface by injecting a lift fluid into the mud.
When drilling an oil or gas well, a starter hole is first drilled and the drilling rig is then installed over the starter hole. Drill pipe is coupled to a drill bit and drill collar, which adds extra weight on the bit, to form the drill string. The drill string is coupled to the kelly joint and rotary table and then lowered into the starter hole. When the drill bit reaches the base of the starter hole, drilling may commence. As drilling progresses, drilling fluid, or mud, is circulated down through the drill pipe to lubricate and cool the drill bit as well as to provide a vehicle for removal of drill cuttings from the borehole. After emerging from the drill bit, the drilling fluid flows up the borehole through the annulus formed by the drill string and the borehole, i.e., the well bore annulus.
In addition to drill bit cooling, lubrication, and cuttings removal, the mud is used for well control. For instance, the mud is used to prevent formation fluid from entering the well bore. When the hydrostatic pressure of mud in the well bore annulus is equal to or greater than the formation pressure, formation fluid will not flow into the well bore and mix with the mud. The hydrostatic pressure of the mud is dependent upon the mud density and the vertical depth. Thus, to prevent formation fluid from flowing into the well bore, the mud is selected based on its density to provide a hydrostatic pressure exceeding the formation pressure. At the same time, however, the hydrostatic pressure of the mud must not exceed the fracture strength of the formation, thereby causing mud filtrate to invade the formation and a filter cake of mud to be deposited on the well bore wall.
As wells become deeper, balancing these two operational constraints becomes increasingly difficult. Moreover, in deep wells more than 30,000 feet below sea level and in water as deep as 10,000 feet, balancing these constraints is not possible because the weight of mud required to produce a hydrostatic pressure exceeding the formation pressure also produces a hydrostatic force exceeding the fracture strength of the formation. When such conditions exist, one solution that allows continued drilling is to case the well bore. Drilling then continues for a time before it is interrupted again and another casing string installed. Drilling then resumes, and so on. Setting multiple casing strings in this manner is, however, very expensive and eventually reduces the diameter of the well bore to the extent that further drilling is not warranted.
Thus, embodiments of the invention are directed to mud return systems that seek to overcome these and other limitations of the prior art.